This invention relates to thin film solar cells and more specifically to an improved transparent conductor contact including a metallic grid.
Considerable research and development has been directed towards producing commercially practical thin film solar cells. In such cells the semiconductor material, typically silicon, is on the order of 0.5 micron thick as compared to conventional single crystal silicon wafers having a thickness on the order of 250 microns. Thus a considerable savings in semiconductor material is achieved by use of thin film structures. Further savings are anticipated because thin film processes are usually more easily automated so that labor costs per watt of output power should be considerably less than that required for the conventional cells.
An example of thin film solar cell structures is illustrated in U.S. Pat. No. 4,292,092 issued to Hanak on Sept. 29, 1981. The Hanak structure employs a glass substrate, a transparent front conductor deposited and patterned on the glass substrate, a thin semiconductor film and a metallic back contact. The performance of such cells is quite dependent upon the sheet resistivity of the transparent conductor which forms a front, light receiving, contact for the active regions of the solar cell. One of the primary teachings of Hanak is a laser scribing technique used to break the sheet of semiconductor material into a series of long narrow strips which are series connected along their edges to add the voltages from individual cells while limiting the maximum current which must be carried by the various conductors. While the teachings of Hanak improve the efficiency of solar cells by reducing internal resistance, resulting modules are not practical for many commercial uses. As indicated by the examples of Hanak, the individual cells are fairly narrow. Thus, on substrates measuring 7.6 centimeters square, ten to twelve individual cells were fabricated. When these teachings are extended to commerical sized modules, for example one by four feet, it can be seen that an extremely large number of individual cells would be fabricated in each module. The large number of cells would not only increase the fabrication cost but would cause the modules to have a very high voltage but low current output. Experience has indicated that module voltages on the order of twelve volts or less are desirable and therefore it is preferred that each module contain no more than about 24 individual cells connected in series.